1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
//! Resolves conditional compilation for [`features` section] in the manifest.
//!
//! This is a [new feature resolver] that runs independently of the main
//! dependency resolver. It has several options which can enable new feature
//! resolution behavior.
//!
//! One of its key characteristics is that it can avoid unifying features for
//! shared dependencies in some situations. See [`FeatureOpts`] for the
//! different behaviors that can be enabled. If no extra options are enabled,
//! then it should behave exactly the same as the dependency resolver's
//! feature resolution.
//!
//! The preferred way to engage this new resolver is via [`resolve_ws_with_opts`].
//!
//! This does not *replace* feature resolution in the dependency resolver, but
//! instead acts as a second pass which can *narrow* the features selected in
//! the dependency resolver. The dependency resolver still needs to do its own
//! feature resolution in order to avoid selecting optional dependencies that
//! are never enabled. The dependency resolver could, in theory, just assume
//! all optional dependencies on all packages are enabled (and remove all
//! knowledge of features), but that could introduce new requirements that
//! might change old behavior or cause conflicts. Maybe some day in the future
//! we could experiment with that, but it seems unlikely to work or be all
//! that helpful.
//!
//! ## Assumptions
//!
//! There are many assumptions made about the dependency resolver:
//!
//! * Assumes feature validation has already been done during the construction
//!   of feature maps, so the feature resolver doesn't do that validation at all.
//! * Assumes `dev-dependencies` within a dependency have been removed
//!   in the given [`Resolve`].
//!
//! There are probably other assumptions that I am forgetting.
//!
//! [`features` section]: https://doc.rust-lang.org/nightly/cargo/reference/features.html
//! [new feature resolver]: https://doc.rust-lang.org/nightly/cargo/reference/resolver.html#feature-resolver-version-2
//! [`resolve_ws_with_opts`]: crate::ops::resolve_ws_with_opts

use crate::core::compiler::{CompileKind, CompileTarget, RustcTargetData};
use crate::core::dependency::{ArtifactTarget, DepKind, Dependency};
use crate::core::resolver::types::FeaturesSet;
use crate::core::resolver::{Resolve, ResolveBehavior};
use crate::core::{FeatureValue, PackageId, PackageIdSpec, PackageSet, Workspace};
use crate::util::interning::InternedString;
use crate::util::CargoResult;
use anyhow::{bail, Context};
use itertools::Itertools;
use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
use std::rc::Rc;

/// The key used in various places to store features for a particular dependency.
/// The actual discrimination happens with the [`FeaturesFor`] type.
type PackageFeaturesKey = (PackageId, FeaturesFor);
/// Map of activated features.
type ActivateMap = HashMap<PackageFeaturesKey, BTreeSet<InternedString>>;

/// Set of all activated features for all packages in the resolve graph.
pub struct ResolvedFeatures {
    activated_features: ActivateMap,
    /// Optional dependencies that should be built.
    ///
    /// The value is the `name_in_toml` of the dependencies.
    activated_dependencies: ActivateMap,
    opts: FeatureOpts,
}

/// Options for how the feature resolver works.
#[derive(Default)]
pub struct FeatureOpts {
    /// Build deps and proc-macros will not share features with other dep kinds,
    /// and so won't artifact targets.
    /// In other terms, if true, features associated with certain kinds of dependencies
    /// will only be unified together.
    /// If false, there is only one namespace for features, unifying all features across
    /// all dependencies, no matter what kind.
    decouple_host_deps: bool,
    /// Dev dep features will not be activated unless needed.
    decouple_dev_deps: bool,
    /// Targets that are not in use will not activate features.
    ignore_inactive_targets: bool,
    /// If enabled, compare against old resolver (for testing).
    compare: bool,
}

/// Flag to indicate if Cargo is building *any* dev units (tests, examples, etc.).
///
/// This disables decoupling of dev dependencies. It may be possible to relax
/// this in the future, but it will require significant changes to how unit
/// dependencies are computed, and can result in longer build times with
/// `cargo test` because the lib may need to be built 3 times instead of
/// twice.
#[derive(Copy, Clone, PartialEq)]
pub enum HasDevUnits {
    Yes,
    No,
}

/// Flag to indicate that target-specific filtering should be disabled.
#[derive(Copy, Clone, PartialEq)]
pub enum ForceAllTargets {
    Yes,
    No,
}

/// Flag to indicate if features are requested for a certain type of dependency.
///
/// This is primarily used for constructing a [`PackageFeaturesKey`] to decouple
/// activated features of the same package with different types of dependency.
#[derive(Default, Copy, Clone, Debug, PartialEq, Eq, Ord, PartialOrd, Hash)]
pub enum FeaturesFor {
    /// Normal or dev dependency.
    #[default]
    NormalOrDev,
    /// Build dependency or proc-macro.
    HostDep,
    /// Any dependency with both artifact and target specified.
    ///
    /// That is, `dep = { …, artifact = <crate-type>, target = <triple> }`
    ArtifactDep(CompileTarget),
}

impl std::fmt::Display for FeaturesFor {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            FeaturesFor::HostDep => f.write_str("host"),
            FeaturesFor::ArtifactDep(target) => f.write_str(&target.rustc_target()),
            FeaturesFor::NormalOrDev => Ok(()),
        }
    }
}

impl FeaturesFor {
    pub fn from_for_host(for_host: bool) -> FeaturesFor {
        if for_host {
            FeaturesFor::HostDep
        } else {
            FeaturesFor::NormalOrDev
        }
    }

    pub fn from_for_host_or_artifact_target(
        for_host: bool,
        artifact_target: Option<CompileTarget>,
    ) -> FeaturesFor {
        match artifact_target {
            Some(target) => FeaturesFor::ArtifactDep(target),
            None => {
                if for_host {
                    FeaturesFor::HostDep
                } else {
                    FeaturesFor::NormalOrDev
                }
            }
        }
    }

    fn apply_opts(self, opts: &FeatureOpts) -> Self {
        if opts.decouple_host_deps {
            self
        } else {
            FeaturesFor::default()
        }
    }
}

impl FeatureOpts {
    pub fn new(
        ws: &Workspace<'_>,
        has_dev_units: HasDevUnits,
        force_all_targets: ForceAllTargets,
    ) -> CargoResult<FeatureOpts> {
        let mut opts = FeatureOpts::default();
        let unstable_flags = ws.config().cli_unstable();
        let mut enable = |feat_opts: &Vec<String>| {
            for opt in feat_opts {
                match opt.as_ref() {
                    "build_dep" | "host_dep" => opts.decouple_host_deps = true,
                    "dev_dep" => opts.decouple_dev_deps = true,
                    "itarget" => opts.ignore_inactive_targets = true,
                    "all" => {
                        opts.decouple_host_deps = true;
                        opts.decouple_dev_deps = true;
                        opts.ignore_inactive_targets = true;
                    }
                    "compare" => opts.compare = true,
                    "ws" => unimplemented!(),
                    s => bail!("-Zfeatures flag `{}` is not supported", s),
                }
            }
            Ok(())
        };
        if let Some(feat_opts) = unstable_flags.features.as_ref() {
            enable(feat_opts)?;
        }
        match ws.resolve_behavior() {
            ResolveBehavior::V1 => {}
            ResolveBehavior::V2 => {
                enable(&vec!["all".to_string()]).unwrap();
            }
        }
        if let HasDevUnits::Yes = has_dev_units {
            // Dev deps cannot be decoupled when they are in use.
            opts.decouple_dev_deps = false;
        }
        if let ForceAllTargets::Yes = force_all_targets {
            opts.ignore_inactive_targets = false;
        }
        Ok(opts)
    }

    /// Creates a new FeatureOpts for the given behavior.
    pub fn new_behavior(behavior: ResolveBehavior, has_dev_units: HasDevUnits) -> FeatureOpts {
        match behavior {
            ResolveBehavior::V1 => FeatureOpts::default(),
            ResolveBehavior::V2 => FeatureOpts {
                decouple_host_deps: true,
                decouple_dev_deps: has_dev_units == HasDevUnits::No,
                ignore_inactive_targets: true,
                compare: false,
            },
        }
    }
}

/// Features flags requested for a package.
///
/// This should be cheap and fast to clone, it is used in the resolver for
/// various caches.
///
/// This is split into enum variants because the resolver needs to handle
/// features coming from different places (command-line and dependency
/// declarations), but those different places have different constraints on
/// which syntax is allowed. This helps ensure that every place dealing with
/// features is properly handling those syntax restrictions.
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub enum RequestedFeatures {
    /// Features requested on the command-line with flags.
    CliFeatures(CliFeatures),
    /// Features specified in a dependency declaration.
    DepFeatures {
        /// The `features` dependency field.
        features: FeaturesSet,
        /// The `default-features` dependency field.
        uses_default_features: bool,
    },
}

/// Features specified on the command-line.
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub struct CliFeatures {
    /// Features from the `--features` flag.
    pub features: Rc<BTreeSet<FeatureValue>>,
    /// The `--all-features` flag.
    pub all_features: bool,
    /// Inverse of `--no-default-features` flag.
    pub uses_default_features: bool,
}

impl CliFeatures {
    /// Creates a new CliFeatures from the given command-line flags.
    pub fn from_command_line(
        features: &[String],
        all_features: bool,
        uses_default_features: bool,
    ) -> CargoResult<CliFeatures> {
        let features = Rc::new(CliFeatures::split_features(features));
        // Some early validation to ensure correct syntax.
        for feature in features.iter() {
            match feature {
                // Maybe call validate_feature_name here once it is an error?
                FeatureValue::Feature(_) => {}
                FeatureValue::Dep { .. } => {
                    bail!(
                        "feature `{}` is not allowed to use explicit `dep:` syntax",
                        feature
                    );
                }
                FeatureValue::DepFeature { dep_feature, .. } => {
                    if dep_feature.contains('/') {
                        bail!("multiple slashes in feature `{}` is not allowed", feature);
                    }
                }
            }
        }
        Ok(CliFeatures {
            features,
            all_features,
            uses_default_features,
        })
    }

    /// Creates a new CliFeatures with the given `all_features` setting.
    pub fn new_all(all_features: bool) -> CliFeatures {
        CliFeatures {
            features: Rc::new(BTreeSet::new()),
            all_features,
            uses_default_features: true,
        }
    }

    fn split_features(features: &[String]) -> BTreeSet<FeatureValue> {
        features
            .iter()
            .flat_map(|s| s.split_whitespace())
            .flat_map(|s| s.split(','))
            .filter(|s| !s.is_empty())
            .map(InternedString::new)
            .map(FeatureValue::new)
            .collect()
    }
}

impl ResolvedFeatures {
    /// Returns the list of features that are enabled for the given package.
    pub fn activated_features(
        &self,
        pkg_id: PackageId,
        features_for: FeaturesFor,
    ) -> Vec<InternedString> {
        self.activated_features_int(pkg_id, features_for)
            .expect("activated_features for invalid package")
    }

    /// Returns if the given dependency should be included.
    ///
    /// This handles dependencies disabled via `cfg` expressions and optional
    /// dependencies which are not enabled.
    pub fn is_dep_activated(
        &self,
        pkg_id: PackageId,
        features_for: FeaturesFor,
        dep_name: InternedString,
    ) -> bool {
        let key = features_for.apply_opts(&self.opts);
        self.activated_dependencies
            .get(&(pkg_id, key))
            .map(|deps| deps.contains(&dep_name))
            .unwrap_or(false)
    }

    /// Variant of `activated_features` that returns `None` if this is
    /// not a valid pkg_id/is_build combination. Used in places which do
    /// not know which packages are activated (like `cargo clean`).
    pub fn activated_features_unverified(
        &self,
        pkg_id: PackageId,
        features_for: FeaturesFor,
    ) -> Option<Vec<InternedString>> {
        self.activated_features_int(pkg_id, features_for).ok()
    }

    fn activated_features_int(
        &self,
        pkg_id: PackageId,
        features_for: FeaturesFor,
    ) -> CargoResult<Vec<InternedString>> {
        let fk = features_for.apply_opts(&self.opts);
        if let Some(fs) = self.activated_features.get(&(pkg_id, fk)) {
            Ok(fs.iter().cloned().collect())
        } else {
            bail!("features did not find {:?} {:?}", pkg_id, fk)
        }
    }

    /// Compares the result against the original resolver behavior.
    ///
    /// Used by `cargo fix --edition` to display any differences.
    pub fn compare_legacy(&self, legacy: &ResolvedFeatures) -> DiffMap {
        self.activated_features
            .iter()
            .filter_map(|((pkg_id, for_host), new_features)| {
                let old_features = legacy
                    .activated_features
                    .get(&(*pkg_id, *for_host))
                    // The new features may have for_host entries where the old one does not.
                    .or_else(|| {
                        legacy
                            .activated_features
                            .get(&(*pkg_id, FeaturesFor::default()))
                    })
                    .map(|feats| feats.iter().cloned().collect())
                    .unwrap_or_else(|| BTreeSet::new());
                // The new resolver should never add features.
                assert_eq!(new_features.difference(&old_features).next(), None);
                let removed_features: BTreeSet<_> =
                    old_features.difference(new_features).cloned().collect();
                if removed_features.is_empty() {
                    None
                } else {
                    Some(((*pkg_id, *for_host), removed_features))
                }
            })
            .collect()
    }
}

/// Map of differences.
///
/// Key is `(pkg_id, for_host)`. Value is a set of features or dependencies removed.
pub type DiffMap = BTreeMap<PackageFeaturesKey, BTreeSet<InternedString>>;

/// The new feature resolver that [`resolve`]s your project.
///
/// For more information, please see the [module-level documentation].
///
/// [`resolve`]: Self::resolve
/// [module-level documentation]: crate::core::resolver::features
pub struct FeatureResolver<'a, 'cfg> {
    ws: &'a Workspace<'cfg>,
    target_data: &'a mut RustcTargetData<'cfg>,
    /// The platforms to build for, requested by the user.
    requested_targets: &'a [CompileKind],
    resolve: &'a Resolve,
    package_set: &'a PackageSet<'cfg>,
    /// Options that change how the feature resolver operates.
    opts: FeatureOpts,
    /// Map of features activated for each package.
    activated_features: ActivateMap,
    /// Map of optional dependencies activated for each package.
    activated_dependencies: ActivateMap,
    /// Keeps track of which packages have had its dependencies processed.
    /// Used to avoid cycles, and to speed up processing.
    processed_deps: HashSet<PackageFeaturesKey>,
    /// If this is `true`, then a non-default `feature_key` needs to be tracked while
    /// traversing the graph.
    ///
    /// This is only here to avoid calling `is_proc_macro` when all feature
    /// options are disabled (because `is_proc_macro` can trigger downloads).
    /// This has to be separate from `FeatureOpts.decouple_host_deps` because
    /// `for_host` tracking is also needed for `itarget` to work properly.
    track_for_host: bool,
    /// `dep_name?/feat_name` features that will be activated if `dep_name` is
    /// ever activated.
    ///
    /// The key is the `(package, for_host, dep_name)` of the package whose
    /// dependency will trigger the addition of new features. The value is the
    /// set of features to activate.
    deferred_weak_dependencies:
        HashMap<(PackageId, FeaturesFor, InternedString), HashSet<InternedString>>,
}

impl<'a, 'cfg> FeatureResolver<'a, 'cfg> {
    /// Runs the resolution algorithm and returns a new [`ResolvedFeatures`]
    /// with the result.
    pub fn resolve(
        ws: &Workspace<'cfg>,
        target_data: &'a mut RustcTargetData<'cfg>,
        resolve: &Resolve,
        package_set: &'a PackageSet<'cfg>,
        cli_features: &CliFeatures,
        specs: &[PackageIdSpec],
        requested_targets: &[CompileKind],
        opts: FeatureOpts,
    ) -> CargoResult<ResolvedFeatures> {
        use crate::util::profile;
        let _p = profile::start("resolve features");
        let track_for_host = opts.decouple_host_deps || opts.ignore_inactive_targets;
        let mut r = FeatureResolver {
            ws,
            target_data,
            requested_targets,
            resolve,
            package_set,
            opts,
            activated_features: HashMap::new(),
            activated_dependencies: HashMap::new(),
            processed_deps: HashSet::new(),
            track_for_host,
            deferred_weak_dependencies: HashMap::new(),
        };
        r.do_resolve(specs, cli_features)?;
        tracing::debug!("features={:#?}", r.activated_features);
        if r.opts.compare {
            r.compare();
        }
        Ok(ResolvedFeatures {
            activated_features: r.activated_features,
            activated_dependencies: r.activated_dependencies,
            opts: r.opts,
        })
    }

    /// Performs the process of resolving all features for the resolve graph.
    fn do_resolve(
        &mut self,
        specs: &[PackageIdSpec],
        cli_features: &CliFeatures,
    ) -> CargoResult<()> {
        let member_features = self.ws.members_with_features(specs, cli_features)?;
        for (member, cli_features) in &member_features {
            let fvs = self.fvs_from_requested(member.package_id(), cli_features);
            let fk = if self.track_for_host && self.is_proc_macro(member.package_id()) {
                // Also activate for normal dependencies. This is needed if the
                // proc-macro includes other targets (like binaries or tests),
                // or running in `cargo test`. Note that in a workspace, if
                // the proc-macro is selected on the command like (like with
                // `--workspace`), this forces feature unification with normal
                // dependencies. This is part of the bigger problem where
                // features depend on which packages are built.
                self.activate_pkg(member.package_id(), FeaturesFor::default(), &fvs)?;
                FeaturesFor::HostDep
            } else {
                FeaturesFor::default()
            };
            self.activate_pkg(member.package_id(), fk, &fvs)?;
        }
        Ok(())
    }

    /// Activates [`FeatureValue`]s on the given package.
    ///
    /// This is the main entrance into the recursion of feature activation
    /// for a package.
    fn activate_pkg(
        &mut self,
        pkg_id: PackageId,
        fk: FeaturesFor,
        fvs: &[FeatureValue],
    ) -> CargoResult<()> {
        tracing::trace!("activate_pkg {} {}", pkg_id.name(), fk);
        // Add an empty entry to ensure everything is covered. This is intended for
        // finding bugs where the resolver missed something it should have visited.
        // Remove this in the future if `activated_features` uses an empty default.
        self.activated_features
            .entry((pkg_id, fk.apply_opts(&self.opts)))
            .or_insert_with(BTreeSet::new);
        for fv in fvs {
            self.activate_fv(pkg_id, fk, fv)?;
        }
        if !self.processed_deps.insert((pkg_id, fk)) {
            // Already processed dependencies. There's no need to process them
            // again. This is primarily to avoid cycles, but also helps speed
            // things up.
            //
            // This is safe because if another package comes along and adds a
            // feature on this package, it will immediately add it (in
            // `activate_fv`), and recurse as necessary right then and there.
            // For example, consider we've already processed our dependencies,
            // and another package comes along and enables one of our optional
            // dependencies, it will do so immediately in the
            // `FeatureValue::DepFeature` branch, and then immediately
            // recurse into that optional dependency. This also holds true for
            // features that enable other features.
            return Ok(());
        }
        for (dep_pkg_id, deps) in self.deps(pkg_id, fk)? {
            for (dep, dep_fk) in deps {
                if dep.is_optional() {
                    // Optional dependencies are enabled in `activate_fv` when
                    // a feature enables it.
                    continue;
                }
                // Recurse into the dependency.
                let fvs = self.fvs_from_dependency(dep_pkg_id, dep);
                self.activate_pkg(dep_pkg_id, dep_fk, &fvs)?;
            }
        }
        Ok(())
    }

    /// Activate a single FeatureValue for a package.
    fn activate_fv(
        &mut self,
        pkg_id: PackageId,
        fk: FeaturesFor,
        fv: &FeatureValue,
    ) -> CargoResult<()> {
        tracing::trace!("activate_fv {} {} {}", pkg_id.name(), fk, fv);
        match fv {
            FeatureValue::Feature(f) => {
                self.activate_rec(pkg_id, fk, *f)?;
            }
            FeatureValue::Dep { dep_name } => {
                self.activate_dependency(pkg_id, fk, *dep_name)?;
            }
            FeatureValue::DepFeature {
                dep_name,
                dep_feature,
                weak,
            } => {
                self.activate_dep_feature(pkg_id, fk, *dep_name, *dep_feature, *weak)?;
            }
        }
        Ok(())
    }

    /// Activate the given feature for the given package, and then recursively
    /// activate any other features that feature enables.
    fn activate_rec(
        &mut self,
        pkg_id: PackageId,
        fk: FeaturesFor,
        feature_to_enable: InternedString,
    ) -> CargoResult<()> {
        tracing::trace!(
            "activate_rec {} {} feat={}",
            pkg_id.name(),
            fk,
            feature_to_enable
        );
        let enabled = self
            .activated_features
            .entry((pkg_id, fk.apply_opts(&self.opts)))
            .or_insert_with(BTreeSet::new);
        if !enabled.insert(feature_to_enable) {
            // Already enabled.
            return Ok(());
        }
        let summary = self.resolve.summary(pkg_id);
        let feature_map = summary.features();
        let Some(fvs) = feature_map.get(&feature_to_enable) else {
            // TODO: this should only happen for optional dependencies.
            // Other cases should be validated by Summary's `build_feature_map`.
            // Figure out some way to validate this assumption.
            tracing::debug!(
                "pkg {:?} does not define feature {}",
                pkg_id,
                feature_to_enable
            );
            return Ok(());
        };
        for fv in fvs {
            self.activate_fv(pkg_id, fk, fv)?;
        }
        Ok(())
    }

    /// Activate a dependency (`dep:dep_name` syntax).
    fn activate_dependency(
        &mut self,
        pkg_id: PackageId,
        fk: FeaturesFor,
        dep_name: InternedString,
    ) -> CargoResult<()> {
        // Mark this dependency as activated.
        let save_decoupled = fk.apply_opts(&self.opts);
        self.activated_dependencies
            .entry((pkg_id, save_decoupled))
            .or_default()
            .insert(dep_name);
        // Check for any deferred features.
        let to_enable = self
            .deferred_weak_dependencies
            .remove(&(pkg_id, fk, dep_name));
        // Activate the optional dep.
        for (dep_pkg_id, deps) in self.deps(pkg_id, fk)? {
            for (dep, dep_fk) in deps {
                if dep.name_in_toml() != dep_name {
                    continue;
                }
                if let Some(to_enable) = &to_enable {
                    for dep_feature in to_enable {
                        tracing::trace!(
                            "activate deferred {} {} -> {}/{}",
                            pkg_id.name(),
                            fk,
                            dep_name,
                            dep_feature
                        );
                        let fv = FeatureValue::new(*dep_feature);
                        self.activate_fv(dep_pkg_id, dep_fk, &fv)?;
                    }
                }
                let fvs = self.fvs_from_dependency(dep_pkg_id, dep);
                self.activate_pkg(dep_pkg_id, dep_fk, &fvs)?;
            }
        }
        Ok(())
    }

    /// Activate a feature within a dependency (`dep_name/feat_name` syntax).
    fn activate_dep_feature(
        &mut self,
        pkg_id: PackageId,
        fk: FeaturesFor,
        dep_name: InternedString,
        dep_feature: InternedString,
        weak: bool,
    ) -> CargoResult<()> {
        for (dep_pkg_id, deps) in self.deps(pkg_id, fk)? {
            for (dep, dep_fk) in deps {
                if dep.name_in_toml() != dep_name {
                    continue;
                }
                if dep.is_optional() {
                    let save_for_host = fk.apply_opts(&self.opts);
                    if weak
                        && !self
                            .activated_dependencies
                            .get(&(pkg_id, save_for_host))
                            .map(|deps| deps.contains(&dep_name))
                            .unwrap_or(false)
                    {
                        // This is weak, but not yet activated. Defer in case
                        // something comes along later and enables it.
                        tracing::trace!(
                            "deferring feature {} {} -> {}/{}",
                            pkg_id.name(),
                            fk,
                            dep_name,
                            dep_feature
                        );
                        self.deferred_weak_dependencies
                            .entry((pkg_id, fk, dep_name))
                            .or_default()
                            .insert(dep_feature);
                        continue;
                    }

                    // Activate the dependency on self.
                    let fv = FeatureValue::Dep { dep_name };
                    self.activate_fv(pkg_id, fk, &fv)?;
                    if !weak {
                        // The old behavior before weak dependencies were
                        // added is to also enables a feature of the same
                        // name.
                        //
                        // Don't enable if the implicit optional dependency
                        // feature wasn't created due to `dep:` hiding.
                        // See rust-lang/cargo#10788 and rust-lang/cargo#12130
                        let summary = self.resolve.summary(pkg_id);
                        let feature_map = summary.features();
                        if feature_map.contains_key(&dep_name) {
                            self.activate_rec(pkg_id, fk, dep_name)?;
                        }
                    }
                }
                // Activate the feature on the dependency.
                let fv = FeatureValue::new(dep_feature);
                self.activate_fv(dep_pkg_id, dep_fk, &fv)?;
            }
        }
        Ok(())
    }

    /// Returns Vec of FeatureValues from a Dependency definition.
    fn fvs_from_dependency(&self, dep_id: PackageId, dep: &Dependency) -> Vec<FeatureValue> {
        let summary = self.resolve.summary(dep_id);
        let feature_map = summary.features();
        let mut result: Vec<FeatureValue> = dep
            .features()
            .iter()
            .map(|f| FeatureValue::new(*f))
            .collect();
        let default = InternedString::new("default");
        if dep.uses_default_features() && feature_map.contains_key(&default) {
            result.push(FeatureValue::Feature(default));
        }
        result
    }

    /// Returns Vec of FeatureValues from a set of command-line features.
    fn fvs_from_requested(
        &self,
        pkg_id: PackageId,
        cli_features: &CliFeatures,
    ) -> Vec<FeatureValue> {
        let summary = self.resolve.summary(pkg_id);
        let feature_map = summary.features();

        let mut result: Vec<FeatureValue> = cli_features.features.iter().cloned().collect();
        let default = InternedString::new("default");
        if cli_features.uses_default_features && feature_map.contains_key(&default) {
            result.push(FeatureValue::Feature(default));
        }

        if cli_features.all_features {
            result.extend(feature_map.keys().map(|k| FeatureValue::Feature(*k)))
        }

        result
    }

    /// Returns the dependencies for a package, filtering out inactive targets.
    fn deps(
        &mut self,
        pkg_id: PackageId,
        fk: FeaturesFor,
    ) -> CargoResult<Vec<(PackageId, Vec<(&'a Dependency, FeaturesFor)>)>> {
        // Helper for determining if a platform is activated.
        fn platform_activated(
            dep: &Dependency,
            fk: FeaturesFor,
            target_data: &RustcTargetData<'_>,
            requested_targets: &[CompileKind],
        ) -> bool {
            // We always count platforms as activated if the target stems from an artifact
            // dependency's target specification. This triggers in conjunction with
            // `[target.'cfg(…)'.dependencies]` manifest sections.
            match (dep.is_build(), fk) {
                (true, _) | (_, FeaturesFor::HostDep) => {
                    // We always care about build-dependencies, and they are always
                    // Host. If we are computing dependencies "for a build script",
                    // even normal dependencies are host-only.
                    target_data.dep_platform_activated(dep, CompileKind::Host)
                }
                (_, FeaturesFor::NormalOrDev) => requested_targets
                    .iter()
                    .any(|kind| target_data.dep_platform_activated(dep, *kind)),
                (_, FeaturesFor::ArtifactDep(target)) => {
                    target_data.dep_platform_activated(dep, CompileKind::Target(target))
                }
            }
        }

        self.resolve
            .deps(pkg_id)
            .map(|(dep_id, deps)| {
                let deps = deps
                    .iter()
                    .filter(|dep| {
                        if dep.platform().is_some()
                            && self.opts.ignore_inactive_targets
                            && !platform_activated(
                                dep,
                                fk,
                                self.target_data,
                                self.requested_targets,
                            )
                        {
                            return false;
                        }
                        if self.opts.decouple_dev_deps && dep.kind() == DepKind::Development {
                            return false;
                        }
                        true
                    })
                    .collect_vec() // collect because the next closure mutably borrows `self.target_data`
                    .into_iter()
                    .map(|dep| {
                        // Each `dep`endency can be built for multiple targets. For one, it
                        // may be a library target which is built as initially configured
                        // by `fk`. If it appears as build dependency, it must be built
                        // for the host.
                        //
                        // It may also be an artifact dependency,
                        // which could be built either
                        //
                        //  - for a specified (aka 'forced') target, specified by
                        //    `dep = { …, target = <triple>` }`
                        //  - as an artifact for use in build dependencies that should
                        //    build for whichever `--target`s are specified
                        //  - like a library would be built
                        //
                        // Generally, the logic for choosing a target for dependencies is
                        // unaltered and used to determine how to build non-artifacts,
                        // artifacts without target specification and no library,
                        // or an artifacts library.
                        //
                        // All this may result in a dependency being built multiple times
                        // for various targets which are either specified in the manifest
                        // or on the cargo command-line.
                        let lib_fk = if fk == FeaturesFor::default() {
                            (self.track_for_host && (dep.is_build() || self.is_proc_macro(dep_id)))
                                .then(|| FeaturesFor::HostDep)
                                .unwrap_or_default()
                        } else {
                            fk
                        };

                        // `artifact_target_keys` are produced to fulfil the needs of artifacts that have a target specification.
                        let artifact_target_keys = dep
                            .artifact()
                            .map(|artifact| {
                                let host_triple = self.target_data.rustc.host;
                                // not all targets may be queried before resolution since artifact dependencies
                                // and per-pkg-targets are not immediately known.
                                let mut activate_target = |target| {
                                    let name = dep.name_in_toml();
                                    self.target_data
                                        .merge_compile_kind(CompileKind::Target(target))
                                        .with_context(|| format!("failed to determine target information for target `{target}`.\n  \
                                        Artifact dependency `{name}` in package `{pkg_id}` requires building for `{target}`", target = target.rustc_target()))
                                };
                                CargoResult::Ok((
                                    artifact.is_lib(),
                                    artifact
                                        .target()
                                        .map(|target| {
                                            CargoResult::Ok(match target {
                                                ArtifactTarget::Force(target) => {
                                                    activate_target(target)?;
                                                    vec![FeaturesFor::ArtifactDep(target)]
                                                }
                                                // FIXME: this needs to interact with the `default-target` and `forced-target` values
                                                // of the dependency
                                                ArtifactTarget::BuildDependencyAssumeTarget => self
                                                    .requested_targets
                                                    .iter()
                                                    .map(|kind| match kind {
                                                        CompileKind::Host => {
                                                            CompileTarget::new(&host_triple)
                                                                .unwrap()
                                                        }
                                                        CompileKind::Target(target) => *target,
                                                    })
                                                    .map(|target| {
                                                        activate_target(target)?;
                                                        Ok(FeaturesFor::ArtifactDep(target))
                                                    })
                                                    .collect::<CargoResult<_>>()?,
                                            })
                                        })
                                        .transpose()?,
                                ))
                            })
                            .transpose()?;

                        let dep_fks = match artifact_target_keys {
                            // The artifact is also a library and does specify custom
                            // targets.
                            // The library's feature key needs to be used alongside
                            // the keys artifact targets.
                            Some((is_lib, Some(mut dep_fks))) if is_lib => {
                                dep_fks.push(lib_fk);
                                dep_fks
                            }
                            // The artifact is not a library, but does specify
                            // custom targets.
                            // Use only these targets feature keys.
                            Some((_, Some(dep_fks))) => dep_fks,
                            // There is no artifact in the current dependency
                            // or there is no target specified on the artifact.
                            // Use the standard feature key without any alteration.
                            Some((_, None)) | None => vec![lib_fk],
                        };
                        Ok(dep_fks.into_iter().map(move |dep_fk| (dep, dep_fk)))
                    })
                    .flatten_ok()
                    .collect::<CargoResult<Vec<_>>>()?;
                Ok((dep_id, deps))
            })
            .filter(|res| res.as_ref().map_or(true, |(_id, deps)| !deps.is_empty()))
            .collect()
    }

    /// Compare the activated features to the resolver. Used for testing.
    fn compare(&self) {
        let mut found = false;
        for ((pkg_id, dep_kind), features) in &self.activated_features {
            let r_features = self.resolve.features(*pkg_id);
            if !r_features.iter().eq(features.iter()) {
                crate::drop_eprintln!(
                    self.ws.config(),
                    "{}/{:?} features mismatch\nresolve: {:?}\nnew: {:?}\n",
                    pkg_id,
                    dep_kind,
                    r_features,
                    features
                );
                found = true;
            }
        }
        if found {
            panic!("feature mismatch");
        }
    }

    fn is_proc_macro(&self, package_id: PackageId) -> bool {
        self.package_set
            .get_one(package_id)
            .expect("packages downloaded")
            .proc_macro()
    }
}